A ventricular extra systole is an ectopic spontaneous ventricular contraction arising in the ventricles and stimulating the myocardium prematurely.
A ventricular extrasystole (VES) with a short coupling interval is frequently followed by a relatively long compensatory pause before returning to presystolic intrinsic ventricular cycle length. Such a so-called short-long sequence has adverse effects on cardiac hemodynamics due to irregular ventricular filling. Moreover, the pause-dependent ventricular instability leads to enhanced dispersion of ventricular refractoriness and thus to a higher vulnerability predisposing to sustained ventricular arrhythmias (Cranefield & Aronson, 1988). Growing evidence has indicated that both monomorphic and polymorphic ventricular tachyarrhythmias can be triggered by the short-long cycles (Denker et al., 1984; Gomes et al., 1989; Roelke et al., 1994; Meyerfeldt et al., 1997, El-Sherif & Turitto, 1999). Meanwhile, preliminary data suggest that suppression of postextrasystolic pauses by overdrive pacing can prevent the onset of ventricular tachyarrhythmias (Viskin et al., 1998, 2000).
Various methods have been proposed for ventricular rate smoothing upon detection of VES. U.S. Pat. No. 4,941,471 issued to Mehra discloses pacemakers designed to stabilize ventricular rate by continuously modulating ventricular escape interval (VEI), which is calculated to be equal to the immediate preceding VEI plus an increment. U.S. Pat. No. 5,545,185 issued to Denker discloses cardiac pacers designed to detect abrupt cycle length shortening, followed by ventricular pacing to prevent the compensatory pause. U.S. Pat. No. 5,814,085 issued to Hill discloses pacemakers designed to improve the patents of Mehra and Denker, in that the VEI is calculated as a function of the underlying heart rate, and optionally as a function of the relative prematurity of the sensed ventricular depolarization. In all the patents cited above, the rate stabilization pacing is applied to right ventricle, without consideration on AV synchronization. U.S. Pat. No. 5,312,451 issued to Limousin et al. discloses an algorithm for the control of both atrial pacing and ventricular pacing after detection of VES. Although this algorithm provides a means to stimulate atrium and ventricle synchronously, the variation of the pacing timer control parameters after VES is not physiological, and the algorithm cannot properly respond to the doublets, triplets, or repeated appearance of VES. U.S. Pat. No. 5,938,687 issued to Bouhour et al. discloses a process that intended to improve the patent of Limousin et al., by incorporating the detection and rhythm control of atrial extrasystole. However, the limitations of the pacing control after VES remain the same.
Although the feature of ventricular rate smoothing has been incorporated in some of the latest dual-chamber cardiac implantable devices to prevent postextrasystolic pause (Viskin et al., 1998; Fromer and Wietholt, 1999; Gronefeld et al., 2002), the existing rate control algorithms are mainly based on ventricular cycle length. As a result, physiologically undesirable pacing effects are likely to occur, for example, retrograde conduction by ventricular pacing without atrial capture (Viskin et al., 1998), and possible very short atrial interval due to competitive atrial pacing. Furthermore, there is clinical evidence that with certain parameter settings (long AV delay, aggressive ventricular rate smoothing, high upper tracking rate), existing ventricular rate smoothing algorithms may result in intradevice interactions that cause device failure or delay detection of sustained monomorphic ventricular tachycardia due to ventricular undersensing (Shivkumar et al., 2000; Glikson et al., 2002; Strohmer et al., 2004).